Mesoporous silica materials are undergoing rapid development for numerous environmental and biomedical applications. These materials are commonly functionalized with small organic molecules through a reaction between an organosilane and the surface silanols. Despite widespread use and implementation of these materials, ligands on their surfaces are challenging to characterize, particularly in aqueous environments. Employing traditional physicochemical characterization methods such as adsorption isotherms, X-ray diffraction, and electron microscopy, as well as solution-phase 1H NMR methods including one-dimensional NMR, diffusion ordered spectroscopy (DOSY) and two-dimensional nuclear Overhauser effect spectroscopy (NOESY), the labile nature of several different surface ligands on mesoporous silica nanoparticles is revealed. The data presented indicate a dynamic model of organosilane release from the surface, and adsorption of the released molecules is ultimately dependent on the nature of the binding of the functional group to the particle surface. A new paradigm for understanding chemical changes that take place at the liquid–solid interface is described, which incorporates a model of chemical dynamics in aqueous solution. Covalently functionalized nanomaterials are widely used, and the characterization of the ligands on their surfaces is of paramount importance, particularly when they are implemented in biomedical and environmental applications.

The surface chemistry of zeolite nanoparticles functionalized with the organosilane aminopropyldimethylmethoxysilane (APDMMS) was selectively probed using solution (1)H NMR spectroscopy. The use of solution NMR spectroscopy results in high-resolution NMR spectra, and the technique is selective for protons on the surface organic functional groups due to their motional averaging in solution. In this study, (1)H solution NMR spectroscopy was used to investigate the interface of the organic functional groups of APDMMS-functionalized silicalite nanoparticles (∼35 nm) in D2O. The pKa for the amine group of APDMMS-functionalized silicalite nanoparticles in D2O was determined using an NMR-pH titration method based on the variation in the proton chemical shift for the alkyl group protons closest to the amine group with pH. The resulting NMR spectra demonstrate the sensitivity of solution NMR spectroscopy to the electronic environment and structure of the surface functional groups.

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